The platelet plasma membrane and cytoplasmic organelles will be investigated by use of freeze etch and freeze fracture techniques. Morphology of endothelial cells of human umbilical cord vein origin will be investigated by means of scanning and transmission electron microscopy. An agent of endothelial cell origin which inhibits platelet aggregation will be studied. The reactivity of platelets and other blood components with commercially available collagen preparation (Avitine) will be studied. An ultrastructural study of the reactivity of platelets and other blood components with the various surfaces of commercially available hemodialysis units and blood tubing will be conducted. In studies with collagen and hemodialysis units, blood from normal subjects and from subjects with various hematologic and blood coagulation factor deficiencies will be used. An ultrastructural study of the earliest morphologic changes which occur in platelets in plasma exposed to various aggregating agents will be undertaken.